This is a hint that some players use, but a lot of players don't, and they should.
When you're attacking a base, you have to kill workers mainly. Workers are the key for a great civilization. Without workers, you can't gather resources. Without resources, you can't build anything. Without buildings or units, you loose.
I was playing against a guy and I killed all his workers. Then, I sent an army to destroy him, and it was really easy, because he hadn't much resources for creating soldiers.
Well, IMO this is the best strategy...